(a) Field of the invention:
The present invention relates generally to a pickup cartridge intended to playback record disks, and it pertains especially to a moving-coil type pickup cartridge functioning in push-pull mode wherein moving coils are arranged to be displaced almost linearly across the magnetic fluxes in magnetic fields produced between opposing magnets.
(b) Description of the prior art:
In those known moving-coil type pickup cartridges wherein the moving coils are designed to be moved substantially linearly, it is generally easy to arrange so that a moving coil is disposed in a narrow air gap defined between magnets which form magnetic fields and that the coil cuts the magnetic flux always perpendicularly to this flux in the air gap. The narrow air gap will ensure the development of higher intensity of magnetic field in the air gap. Because of this higher intensity of the magnetic field and the perpendicular cutting of the magnetic fluxes by the moving coil, such known arrangement of the moving coil has the inherent advantage to provide a higher efficiency of movement-electromagnetism conversion and a high output voltage. This known coil arrangement will therefore allow its formation on a light bobbin of any non-magnetic material rather than on a heavy bobbin of magnetizable material, without the accompaniment of a considerable decrease in the conversion efficiency. This fact will serve to decrease the effective mass of a vibration system in the pickup cartridge, improving mechanical characteristics of the pickup cartridge, such as compliance, trackability or the like.
However, the above-mentioned advantages are not completely materialized in the prior art pickup cartridges, as will be discussed below.
A typical arrangement of the movement-electromagnetism conversion system according to the prior art is conceptually shown in FIG. 1, with the moving coil 1 being situated at its neutral position. As noted in FIG. 1, only upper half 1a of the moving coil 1 is located within the magnetic field 2, and the remaining lower half 1b resides outside this magnetic field 2. In accordance with the movement of the cantilever (not shown) in the pickup cartridge, the moving coil 1 is caused to move substantially linearly in a plane perpendicular to the magnetic flux which is generally indicated at B in the magnetic field 2, and more particularly in such manner that said one half 1a cuts the magnetic flux B inducing an electromotive force corresponding to the movement of the moving coil 1. With this arrangement, however, the two halves of the moving coil 1 have different characteristics of displacement-electromagnetism conversion relative to each other so that the induced signal contains undesired asymmetrical distortion. Also, this known arrangement has another inconvenience due to the leakage magnetic flux around the magnetic field 2. Namely, the magnetic field 2 is usually formed between two opposite magnet poles separated via an air gap; and around these poles, there always exist some leakage magnetic flux. Therefore, it is not possible to define the boundary of the magnetic field 2 by a straight line. Thus, actual magnetic field distribution in the movement-electromagnetism conversion system may take the pattern as shown in FIG. 2, wherein the shaded part represents a leakage magnetic field formed outside the magnetic field 2 due to the leakage magnetic flux. The moving coil 1, when cutting the leakage flux, induces an electromotive force in its lower half 1b which is opposite in polarity to that of the electromotive force in its upper half 1a. As a result, the level of the output signal is decreased. Furthermore, the intensity of the leakage magnetic field varies with the distance from the magnet, and the aimed magnetic field 2 is affected by the varying leakage magnetic field so that the intensity of the aimed magnetic field 2 will change non-linearly, particularly in the boundary region as shown in FIG. 2. Consequently, the output signal from the moving coil 1 is susceptible to present non-linear distortions due to the aforementioned non-uniformity of the magnetic field. Needless to say, it is impossible to locate the whole moving coil 1 within a common uniform magnetic field, because, if so arranged, electromotive forces induced in the two halves of the moving coil 1 will cancel each other to deliver no output signal.
Another arrangement of the movement-electromagnetism conversion system according to the prior art, which is disclosed in the laid open Japanese Utility Model Application No. 52-73703, is schematically illustrated in FIGS. 3 and 4, wherein the moving coil 9 is shown to be situated at its neutral position. The moving coil 9 is arranged on a coil support plate 7 extending from a cantilever 8 into an air gap defined between a pair of permanent magnets 5a and 5b. Each of the permanent magnets 5a and 5b is magnetized so as to have an N-pole at its middle portion and S-poles at its extremity portions. Therefore, two opposite-direction magnetic fields 6a and 6b are generated along the inner surfaces of the magnets 5a and 5b, as shown. As most clearly depicted in FIG. 4, one half 9a of the moving coil 9 is associated with flux B.sub.1 in the magnetic field 6a, and the other half 9b is associated with flux B.sub.2 in the magnetic field 6b.